1. Field of the Invention
The invention relates to the field of optical devices, in particular liquid crystal imagers, as well as the mirrors associated with these optical devices.
The preferred field of application of the invention is the field of liquid crystal imagers, also referred to as LCD imagers (LCD being the abbreviation for “liquid crystal display”. Liquid crystal imagers are generally illuminated through the rear of the imager, the front of the imager being occupied by the liquid crystal screen. This illumination is generally carried out using lamps, often tube lamps, which lamps are advantageously, on the one hand, positioned parallel to the liquid crystal screen, and, on the other hand, accommodated in an optically matched support, which support is referred to as a light box. The number and size of the lamps vary as a function of the size of the liquid crystal screen and the illumination uniformity required by the particular application in question.
The invention applies particularly to the liquid crystal imagers used in aircraft, airplanes or helicopters. This is because the liquid crystal imagers of military aircraft have a day illumination stage producing intense and relatively uniform illumination with the aid of numerous lamps, also referred to as day lamps, as well as a night illumination stage producing weaker illumination, scattered more uniformly and needing to be infrared-filtered, particularly in order to avoid hampering or blinding pilots who are wearing certain equipment such as night vision goggles, the night illumination being carried out with the aid of one or two lamps, also referred to as night lamps. The day lamps and the night lamps generally operate mutually exclusively. The day lamps are placed as close as possible to the liquid crystal screen, so as to illuminate this screen as directly as possible with a maximum intensity. The general direction of the light emitted by the night lamps will consequently be less direct. The invention therefore relates to a liquid crystal imager as well as the mirror which is associated with it, this mirror being placed in the path of the light which comes from the night lamps and which will illuminate the liquid crystal screen.
2. Description of the Prior Art
A first prior art provides an optical device, and more precisely a liquid crystal imager in which, on the one hand, the day lamps are located just behind the liquid crystal screen, and, on the other hand, the night lamp or lamps are located at the bottom of the light box, behind the day lamps, an infrared filter being arranged between the day lamps and the night lamps.
FIG. 1 schematically represents a view in section of a liquid crystal imager according to the first prior art. A light box 1 is closed by a liquid crystal screen 5. The light box 1 is separated into two compartments by an infrared filter 4. There are a plurality of day lamps 2 in the front part of the light box 1, i.e. between the liquid crystal screen 5 and the infrared filter 4. There are one or more night lamps 3 in the rear part of the light box 1, i.e. between the infrared filter 4 and the bottom of the light box 1. Dot-and-dash arrows represent light rays coming from the day lamps 2. Dashed arrows represent light rays coming from the night lamps 3.
The first prior art has several drawbacks. One drawback of the first prior art relates to the significant depth P of the light box 1. Another drawback of the first prior art involves the large dimensions of the infrared filter 4, which are of the same order of magnitude as those of the liquid crystal screen 5, which dimensions may be, for example, typically as much as ten or so centimeters on a side. The infrared filter 4 consists of processed glass with a small thickness, however, which makes it relatively fragile, expensive and difficult to cut. Another drawback of the first prior art is due to the fact that the efficiency of the light stage is impaired by the presence of the infrared filter 4, which is located just behind the day lamps 2 and by which the received light coming from the day lamps 2 is not reflected toward the liquid crystal screen 5. When the liquid crystal screen 5 is of rectangular rather than square shape, another drawback of the first prior art is due to the length difference between the day lamps 2 and the night lamps 3, leading to two separate component references, one per type of lamp, which tends to increase the number of component references for the liquid crystal imager. Another drawback of the first prior art is that of creating light halos around the night lamp or lamps 3.
A second prior art provides an optical device, and more precisely a liquid crystal imager in which, on the one hand, the day lamps are arranged at the bottom of the light box, and, on the other hand, a night lamp is fitted at the bottom of a separate and filtered channel, so as to illuminate the end of an optical prism which is designed so as to be able to scatter the light coming from the night lamp uniformly over the entire surface of the liquid crystal screen.
FIG. 2 schematically represents a view in section of a liquid crystal imager according to the second prior art. A light box 1 is closed by a liquid crystal screen 5. There are a plurality of day lamps 2 in the rear part of the light box 1, i.e. close to the bottom 10 of the light box 1. The bottom 10 of the light box 1 is reflective so that the light emitted by the day lamps 2 toward the rear of the light box 1 can be reflected by it toward the liquid crystal screen 5. On one side of the light box 1, behind the reflective bottom 10, there is a night lamp 3 located at the bottom of a channel 9. Dot-and-dash arrows represent light rays coming from the day lamps 2. Dashed arrows represent light rays coming from the night lamp 3. An optical prism 6 is placed between the liquid crystal screen 5 and the day lamps 2, some of the optical prism 6 being located at the exit of the channel 9. The shape of the optical prism 6 is such that at least some of the light being emitted by the night lamp 3, and having passed through the channel 9, firstly passes through the face 61 of the optical prism 6, then is reflected by the face 62 of the optical prism 9, and subsequently arrives at the face 63 of the prism before finally being scattered in the direction of the liquid crystal screen 5 by a diffuser 8 located against the face 63. The optical prism 6 is designed so as to scatter the light arriving at its face 63 uniformly over the entire surface of the liquid crystal screen 5 by means of the diffuser 8. A layer 7 of adhesive, located against the face 64 of the optical prism 6, helps secure this optical prism 6 to the light box 1. An infrared filter 4, located against the face 61 of the optical prism 6, makes it possible to filter the light coming from the night lamp 3 before it arrives at the liquid crystal screen 5.
The second prior art has several drawbacks. One drawback of the second prior art relates to the use of an optical prism 6, which is made of heavy and fragile glass. Another drawback of the second prior art involves the production of the optical prism 6, since this production is difficult and expensive. Another drawback of the second prior art, due to the use of the optical prism 6 for the night stage, is the limitation of the performance of the day stage of the optical imager, since the optical prism remains permanently between the liquid crystal screen 5 and the day lamps 2. Another drawback of the second prior art results from the actual shape of the optical prism 6, which prevents a second night lamp 3 from being installed on the other side of the light box 1, thus preventing the production of an imager with large dimensions containing a liquid crystal screen 5 likewise with large dimensions; in fact, this type of liquid crystal screen 5 with large dimensions often requires the use of two night lamps 3 in order to be illuminated sufficiently uniformly by scattering and with a sufficient intensity. An imager with large dimensions is, for example, an imager having a liquid crystal screen in the form of a square measuring more than ten or so centimeters per side, for example twenty or so centimeters per side.
A third prior art provides an optical device, and more precisely a liquid crystal imager in which, on the one hand, the day lamps are arranged at the bottom of the light box, and, on the other hand, one or two night lamps are each fitted on the side of the light box, so as to illuminate the side-edge of a night light guide. This night light guide is substantially flat, and is structured so that the light which arrives perpendicularly to one and/or the other of its side-edges can be returned by it perpendicularly to one of its faces. This light guide may also be used in an imager according to the invention. An example of such a night light guide is represented in FIG. 3.
FIG. 3 schematically represents a view in perspective of an element belonging to a liquid crystal imager according to a third prior art as represented in FIG. 4, said element being a light guide. This light guide may also be used in the scope of the invention. The light guide is in the form of a substantially flat element, which is structured so that the light received perpendicularly to one and/or the other of its side-edges 110 is returned by it substantially perpendicularly to one of its faces, for example the face 111. The propagation of the light is represented here by dashed arrows. The structure of the light guide is a particular one so that the light received perpendicularly to one of its side-edges 110 can be returned by it perpendicularly to its face 111, by scattering or by diffraction. Here, for example, the face 111 is striated, and has a set of mutually parallel striations 112. Instead of having striations on one of its faces, the light guide may have microprisms obtained, for example by hot compression. The material of the light guide is, for example, PMMA, i.e. plexiglas (registered trademark), because this has very good optical transparency. The type of light guide used in the imager according to the third prior art, or according to the invention, is for example a “flat collimator” from “Briteview Technologies” (registered trademark) which is described in particular in a document entitled “A1.3: flat collimator: a backlighting assembly utilizing microprisms for high energy efficiency” written by “C-Y. TAI & H. ZOU” and published in “SID 94 Applications Digest”.
FIG. 4 schematically represents a view in section of a liquid crystal imager according to a third prior art. A light box 1 is closed by a liquid crystal screen 5. There are a plurality of day lamps 2 in the rear part of the light box 1, i.e. close to the bottom 10 of the light box 1. The bottom 10 of the light box 1 is reflective so that the light emitted by the day lamps 2 toward the rear of the light box 1 can be reflected by it toward the liquid crystal screen 5. Between the liquid crystal screen 5 and the day lamps 2, there is the light guide 11 described, for example, in FIG. 3. On each side of the light box 1, there is a night lamp 3 arranged facing one of the side-edges 110 of the light guide 11. The face 111 of the light guide 11 is arranged parallel to the surface of the liquid crystal screen 5. There is an infrared filter 4 between each of the lamps 3 and each of the side-edges 110 of the light guide 11. Dot-and-dash arrows represent light rays coming from the day lamps 2. Dashed arrows represent light rays coming from the night lamp 3. Some of the light emitted by the day lamps 2 passes through the light guide 11 before arriving at the liquid crystal screen 5. Some of the light emitted by the night lamps 3 passes through the infrared filter 4, and enters the light guide 11 through the side-edge 110 of this guide 11 before emerging through the face 111 of the same guide 11, in order to arrive at the liquid crystal screen 5.
A drawback of the third prior art is that it has too large a length L of the imager, due to the successive presence over the length L of a night lamp 3, an infrared filter 4, a light guide 11, and optionally another infrared filter 4 and another night lamp 3.
The problem of the first and second prior arts is that of having a number of drawbacks, while the problem of the third prior art is that of having one major drawback, particularly for certain applications such as liquid crystal imagers.